In
quantum mechanics, the procedure of constructing
eigenstates of total angular momentum out of eigenstates of separate angular momenta is called
angular momentum coupling. For instance, the orbit and spin of a single particle can interact through
spin–orbit interaction, in which case the complete physical picture must include spin-orbit coupling. Or two charged particles, each with a well-defined angular momentum, may interact by Coulomb forces, in which case coupling of the two one-particle angular momenta to a total angular momentum is a useful step in the solution of the two-particle
Schrödinger equation. In both cases the separate angular momenta are no longer
constants of motion, but the sum of the two angular momenta usually still is. Angular momentum coupling in atoms is of importance in atomic
spectroscopy. Angular momentum coupling of
electron spins is of importance in
quantum chemistry. Also in the
nuclear shell model angular momentum coupling is ubiquitous.